The use of digested, water-soluble starches and of water-soluble starch derivatives as assistants in papermaking, in particular for increasing the strength of the paper, is described generally in Ullmanns Enzyklopadie der Technischen Chemie, 4th edition, Verlag Chemie, Weinheim, 1979, vol. 17, page 581 et seq. However, the retention of the dissolved starches in the paper is poor, leading to considerable pollution of the waste waters.
U.S. Pat. No. 3,734,802 describes the use of cationic dextran graft copolymers as additives for improving the dry strength of paper. The graft copolymers are added to an aqueous cellulose dispersion. This mixture is then used for the production of paper sheets. The graft copolymers are prepared by grafting dextran, a naturally occurring polymer having an average molecular weight of from about 20,000 to about 50,000,000, with cationic monomers, for example diallylic, quaternary ammonium compounds, such as diallydimethylammonium chloride, or acrylic, quaternary ammonium compounds such as dimethylaminoethyl methacrycate, and mixtures thereof with acrylamide or methacrylamide. The graft copolymerization is preferably carried out in the presence of a redox catalyst, for example one based on cerium.
U.S. Pat. No. 3,467,608 describes a process for the preparation of a cationic starch by heating an aqueous starch suspension with the addition of a polyalkyleneimine, such as polyethyleneimine, or a polyalkylene polyamine, such as polyethylenepolyamine, the molecular weight of the polymer used for the modification being at least 50,000. The reaction mixture contains from 0.5 to 40% by weight of the polymer and from 60 to 99.5% by weight of starch, percentages being based on solids content. It is true that the possibility of using modified starches obtainable by hydrolysis, oxidation, esterification and etherification is also described generally. However, the publication gives no indication of the use of anionically modified starches. All preferred embodiments relate to unmodified potato and corn starch. The resulting cationic starch derivatives are used as flocculants.
U.S. Pat. No. 4,097,427 describes a process for the conversion of starch into cationic form, an aqueous starch suspension being reacted under alkaline reaction conditions with a water-soluble polymer which contains quaternary ammonium groups in the presence of an oxidizing agent. Exclusively unmodified or slightly degraded starches are used for the modification, preferably corn starch and tapioca starch. The polymers containing quaternary ammonium groups are selected from polymers having epichlorohydrin units, quaternized polyethylene imines, quaternized diallyldialkylamine polymers etc. The oxidizing agents used are, for example, ammonium persulfate, hydrogen peroxide, sodium hypochlorite, ozone or tert-butyl hydroperoxide. The pH of the starch suspension is brought to about 8 with water-soluble bases. The modified cationic starches thus prepared are added to the paper stock as strength agents during papermaking. In this process, however, the waste water has a very high COD value (COD=chemical oxygen demand).
U.S. Pat. No. 4,146,515 describes a process for the preparation of a gelatinous, cationic starch for the surface sizing and coating of cellulosic material. An aqueous suspension of a partially oxidized starch is digested together with a cationic polymer in a continuous digester The cationic polymers used are condensates of epichlorohydrin and dimethylamine, polymers of diallyldimethylammonium chloride, quaternized reaction products of vinylidene chloride and ammonia, quaternized polyethylene imine and quaternized polyepichlorohydrin. The starches used are starches which have been partially oxidized, for example with sodium hypochlorite and which are also ambiguously referred to as anionic starches in the publication. The publication gives no indication of a possible modification of starch esters and starch ethers usually referred to as anionic starches.
DE-A-37 19 480 and EP-A-0 282 761 describe a process for the production of paper, board and cardboard having high dry strength by the addition of a dry strength agent comprising a mixture of cationic polymers and starch to the paper stock. The cationic polymers contain, as characteristic monomers, polymerized units of diallyldimethylammonium chloride, N-vinylamine or N-vinylimidazoline. Polyethylene imine may also be used as the cationic polymer. For the preparation of the dry strength agents, either an aqueous suspension of a natural potato starch is digested in the presence of the polymers by heating to above the glutinization temperature in the absence of oxidizing agents, polymerization initiators and alkalis or a digested potato starch is reacted with the cationic polymers at from 15 to 70.degree. C. Exclusively natural or thermally degraded potato starch is used and the modification with the cationic polymers is carried out in aqueous suspension or in aqueous solution.
EP-A-0 301 372, makes a similar disclosure, cationically modified, enzymatically degraded starchs being used.
U.S. Pat. No. 4,880,497 and U.S. Pat. No. 4,978,427 describe water-soluble copolymers which contain vinylamine units and their use as wet strength agents and dry strength agents for paper. The polymers are prepared by copolymerizing N-vinylformamide and an ethylenically unsaturated monomer selected from vinyl acetate, vinyl propionate, alkyl vinyl ethers, esters, nitrites and amides of acrylic and methacrylic acid and N-vinylpyrrolidone and subsequently hydrolyzing from 30 to 100 mol % of the formyl groups of the resulting copolymer with formation of amino groups. The hydrolyzed copolymers are used for papermaking in amounts of from 0.1 to 5% by weight, based on dry fibers.
EP-A-0 418 343 likewise describes a process for the production of paper, board and cardboard by draining a paper stock in the presence of polymers containing N-vinylformamide units. Unhydrolyzed copolymers which contain as the polymerized units N-vinylformamide and at least one further monomer containing cationic groups are used. These additional monomers are selected from the group consisting of the (meth)acrylamides having alkyleneammonium radicals or from the group consisting of the diallyldialkylammonium compounds. In order to achieve good retention with respect to paper fibers when these copolymers are used as dry strength agents for paper, an aqueous solution prepared beforehand by heating the aqueous copolymers together with natural potato starch to above the glutinization temperature of the starch in the absence of oxidizing agents, polymerization initiators and alkalis is added to the paper stock.
WO 96/13525 describes a process for the cationic modification of starch by reacting various natural starches and wax starches with polymers which contain amino and/or ammonium groups. The reaction is carried out in aqueous solution at elevated temperatures and superatmospheric pressure in the absence of oxidizing agents, polymerization initiators and alkalis, the starch being sufficiently digested and only a slight molecular weight decrease of not more than 10% by weight of the starch occurring. The cationic starches thus obtained are used as dry strength agents in papermaking.
The properties of the abovementioned starch derivatives modified with cationic polymers are worthy of improvement, in particular with respect to their use as assistants for papermaking. Thus, good retention of the respective dry strength agent by the cellulose fibers is required for achieving good mechanical strength of the product in the dry state. It is an object of the present invention to provide cationically modified starches which have improved starch retention by the paper fibers and, when used as dry strength agents, therefore give a paper product having good mechanical properties.